Today Congress continues hearings with social media giants about Russian interference in U.S. elections. Lawmakers are not the only ones worried. People in Silicon Valley - tech investors, startup founders - are, too. NPR's Aarti Shahani spoke with a man who mentored CEO of Facebook Mark Zuckerberg, and he wants Capitol Hill to regulate Facebook.

AARTI SHAHANI, BYLINE: Roger McNamee is managing director of the investment firm Elevation Partners. More importantly, he's the guy who told Mark Zuckerberg to never sell Facebook. In the spring of 2006, he recalls, the young CEO walked into his office to get career advice.

ROGER MCNAMEE: So I said, look, here's the deal. I believe you've created the most important company since Google and that you will before long be bigger than Google is today.

SHAHANI: McNamee takes credit for helping convince Zuckerberg to stay the course, not sell out to a bigger tech company, and for mentoring him and introducing him to Sheryl Sandberg, the woman credited with turning the startup into a grownup. Now let's fast forward a decade. Shortly before the 2016 election, before Capitol Hill's alarm bells went off, McNamee says he noticed nasty memes about Black Lives Matter and Brexit spreading like wildfire on Facebook. They looked manufactured, the work of organized groups, not regular users.

MCNAMEE: And I, literally, I'm starting to compile a list. And I have, like, 15 or 16 examples.

SHAHANI: He emailed Zuckerberg and Sandberg, but he says their response is not what he'd hoped. Instead of openly exploring what structural problems might be at play - bots pretending to be humans or criminals exploiting the easy-to-use advertising platform - they passed them off to a VP who handles marketing and partnerships. It was a PR response, he says.

MCNAMEE: And they keep saying two things. Roger, you don't understand. We're doing all this great stuff you can't see. And, secondly, we're not responsible for what third parties do on our platform.

(SOUNDBITE OF ARCHIVED RECORDING)

AL FRANKEN: You can't put together rubles with a political ad and go like, those two data points spell out something bad?

SHAHANI: This is Senator Al Franken at yesterday's Senate hearing asking why alarm bells didn't go off at Facebook when Russian currency was used to buy U.S. political ads. Facebook lawyer Colin Stretch responds.

(SOUNDBITE OF ARCHIVED RECORDING)

COLIN STRETCH: Senator, it's a signal we should have been alert to, and, in hindsight, it's one we missed.

FRANKEN: OK, OK. Yeah.

SHAHANI: Roger McNamee has been taking trips to Washington, D.C., to speak with lawmakers and urge them to regulate the social media giants, hold them responsible for the content running through their pipes. Today lawyers from Facebook, Twitter and Google go before Congress again. Zuckerberg will not attend. He's got a quarterly-earnings call with investors. McNamee says lawmakers should make the CEO come and testify.

MCNAMEE: Facebook in particular has taken this very extreme, essentially libertarian position that it's not responsible for the actions that other people take with their tools. That position, I believe, will be very hard to defend in an open hearing. It would be healthy for everyone to watch Mark try to defend it.

SHAHANI: Facebook says the company is hiring more than 4,000 new people to help clean up bots and illegal activity. It will begin requiring identity documents for some political advertisers and criminal activity playing out on the site is a matter for law enforcement. Aarti Shahani, NPR News, San Francisco.